<head>
    <title>margin 内部高度撑开</title>
    <link rel="stylesheet" href="../style.css">
</head>
<div class="box w200 bg-pink">
    <div class="bg-blue w100 h100"></div>
</div>
<style>
    .box {
        /* 撑开高度：添加边框或触发 BFC */
        overflow: hidden;
    }
    .box div {
        margin: 100px auto 0;
    }
</style>
